1. Field of the Invention
The present invention relates to an electrical tool, and, in particular, to a manually operated sander such as an orbital sander or the like including a motor-driven grinding plate adjacent to a workpiece, and a device for releasably clamping the abrasive paper to the grinding plate, with the clamping device having a clamping surface element for securing at least one end region of the abrasive paper.
2. Description of the Prior Art
In a sander of the type described above, the grinding plate is driven by an electric motor which oscillates the grinding plate over the workpiece. An abrasive paper, which is secured to the bottom of the grinding plate with the clamping device, removes, as a result of the oscillating movement of the grinding plate, the material from the surface of the workpiece. The service life of the abrasive paper depends on the material of the workpiece and prevailing basic condition. After the expiration of the service life of the abrasive paper, the worn-out paper should be replaced with a new one. For replacing the worn-out abrasive paper, the clamping device, which is provided on the upper surface of the grinding plate is actuated in order to release the worn-out abrasive paper and to provide for its replacement. Then, the clamping device clamps the new abrasive paper to the grinding plate. In the clamping condition of the new abrasive paper, at least the end region of the abrasive paper is located in the clamping device in the clamping condition of the abrasive paper. A concave clamping surface of the clamping device forms a bearing surface for the clamping section of the abrasive paper.
An orbital sander of the above-described type is disclosed in German Publication DE 198 00 04 A1. The known sander includes a grinding plate and a clamping device for securing an abrasive paper to the grinding plate. The clamping device has a clamping surface that cooperates with a roll-shaped, elastic clamping stirrup. At least one end region of the abrasive paper is introduced between the clamping stirrup and the clamping surface and is clamped therebetween. The necessary clamping force is achieved by clamping the clamping stirrup against the clamping surface.
The arrangement disclosed in DE 198 00 044 A1 permits to satisfactory secure the abrasive paper to the grinding plate with the clamping device. However, the drawback of the arrangement of DE 198 00 044 A1 consists in that the abrasive paper, while being adequately clamped by the clamping device, is not tensioned. As a result, the quality of grinding as well as the service life of the abrasive paper are reduced. The reduction in the service life necessitates often replacement of the abrasive paper. Alternatively, additional time may be required for tensioning of the abrasive paper when the known clamping device is used.
Further, the use of abrasive papers having different thicknesses and graining makes the handling of the sander more difficult and, in particular, the handling of the clamping device with which different operational forces need be applied, as there is no possibility to regulate the pressure force.
Moreover, the resiliency of the clamping stirrup is subjected to changes which results in changes of the pressing force applied thereby. The user of the sander has no means available for compensating the pressing force changes and which permits to avoid the changes of the pressing force.
Accordingly, an object of the present invention is to provide a sander with a clamping device which would insure a reliable clamping of the abrasive paper and, simultaneously, tensioning of the abrasive paper.
Another object of the present invention is to provide a sander with clamping device which would insure easy handling even when abrasive papers having different thicknesses are used.
A further object of the present invention is to provide a sander with a clamping device which can be economically produced.
This and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a clamping device with a clamping stirrup eccentrically pivotable with respect to the clamping surface element, with a gap between a pivotal path of the clamping stirrup and the clamping surface element substantially closing toward a region of the clamping surface element that cooperates with the claiming stirrup for clamping the end region of the abrasive paper to the grinding plate.
The pivotal arrangement of the clamping stirrup and a gap between the clamping surface and the clamping stirrup, the width of which is gradually reduced, permit to obtain a clamping force that would provide for at least clamping of the end region of the abrasive paper. Further, the invention facilitates the use of abrasive papers having different thicknesses.
Moreover, the foregoing novel features of the present invention provide for compensation of changes in the clamping characteristics of the clamping device. Still further, the pivotal arrangement of the clamping stirrup and the gap between the clamping surface and the stirrup permit to increase tolerances of the parts of the clamping device, which reduces the production costs. Overall, the present invention provides a sander and, in particular, a clamping device that is very easy to handle.
Advantageously, the clamping stirrup has a circular pivotal path. The circular pivotal path insures an easy conversion of the clamping device and, in particular, positioning of the clamping stirrup. With the clamping stirrup being pivotally supported on the grinding plate, it can be easily guided. The clamping device can be easily operated with one hand. This significantly facilitates handling of the entire sander.
Advantageously, the pivotal angle, which is described by the clamping stirrup upon its pivotal movement into the clamping position, amounts to at least 45xc2x0, preferably, to about 90xc2x0. A wide pivotal region provides for a greater clamping path. A greater or longer clamping path insures an optimal tensioning or tightening of the used abrasive paper and reduces expenditure of forces necessary for clamping of the abrasive paper.
According to a preferred embodiment of the present invention, the clamping surface is formed as a circular or arcuate surface which further simplifies the manufacturing process, making it more economical.
To provide for a simple insertion of the abrasive paper, the clamping surface element preferably is provided in a region of an edge of the grinding plate remote from the workpiece. With such an arrangement of the clamping surface element(s), the abrasive paper becomes pressed against a side of the grinding plate adjacent to the workpiece, with one or both end region(s) of the abrasive paper being clamped between the clamping surface element(s) and the clamping stirrup(s). All this further facilitates handling of the sander. Naturally, the clamping device can be provided at another location of the grinding plate.
In the preferred embodiment of the sander according to the present invention, the clamping means includes two clamping devices each having a clamping surface element and a clamping stirrup which are provided at two opposite, remote from the workpiece, edges of the grinding plate.
Two U-shaped spring members, which are advantageously provided at opposite ends of the clamping stirrup, prevent the clamping force from exceeding a predetermined threshold value. The provision of the spring members also permits to increase the tolerance range of the sander parts during manufacturing of the clamping device.
To insure a cost-effective manufacturing of the clamping device, the clamping stirrup and the spring members are formed as a one-piece part made of wire.
Advantageously, the clamping surface element is formed of a sheet metal or of any other hard material in order to insure optimal sliding of the abrasive paper over the clamping surface. This insures optimal tensioning of the abrasive paper because the abrasive paper slides well over the hard surface, whereby it is insured that the abrasive paper firmly abuts the side of the grinding plate adjacent to the workpiece.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiment, when read with reference to the accompanying drawings.